At present the most widely accepted technique for applying aural caloric irrigations for determining disordered vestibular function employs four separate aural irrigations with water at a high flow rate, the water temperature being carefully controlled so as to be at 7.degree. C above and below body temperature. In this technique, all stimulations are of equal strength, in order to detect and measure small differences in excitability between labyrinths, as well as between right-beating versus left-beating nystagmus. This technique has helped to advance the study of vestibular disorders, but further improvements are needed to overcome certain objectionable features of said technique. One problem with said technique is the prolonged duration of each stimulation, which is often stressful to patients and which results in a requirement of long waiting periods between irrigations. Originally it was recommended that a waiting period of at least 9 minutes should separate the onset of successive irrigations, but more recently a 30-minute waiting period has been advised when precise measurements of vestibular responsiveness are desired.
The prolonged duration of the caloric stimulus is caused by a slow exchange of heat between the warmed or cooled labyrinth and surrounding tissues and material, such as blood, temporal bone and skin of the aural canal. With regard to the rate of heat exchange, data obtained from intralabyrinthine temperature measurements made in living human subjects show that temperature disturbances in the labyrinth area caused by conventional caloric irrigations may last for over 10 minutes. According to studies made, it would appear that a weak but persistent caloric stimulus continues to act during the waiting period between conventional irrigations, and may still be present when the next irrigation is commenced. The cessation of nystagmus which occurs in the face of such a persisting caloric stimulus may be the result of an adaptation of the vestibular neural elements. It is possible, therefore, that a residual caloric stimulus may interfere with the results of successive conventional irrigations through a modification of vestibular responsiveness induced by such a prolonged action. Furthermore, most clinicians assess the intensity of the nystagmic response by measuring nystagmus at the peak of the reaction, and therefore a prolonged stimulation would not seem to be required for evaluating vestibular responsiveness.
Another problem with the above-mentioned technique is that there appears to be no convenient means for adjusting the intensity of the stimulus.